How can i skin a model to a pose other than the Rest Pose?
I would suggest this: in the rig tab of your armature, create a pose library, and next a pose wich will hold the rest pose of your rig.

Next pose the rig as you like, make it the rest pose, work, and when you're done, restore the rest pose from the library.

BTW, maybe you can even make simpler: in the mesh object, modifiers tab, you can activate the 'Edit' button of the Armature modifier. Then you can pose your model and work on it, not only in 'Edit' mode, but 'Weight paint' too.

EDIT For instance, to rig a character already posed (i.e. being not standing straight), you can:


create a standard rig.
In pose mode, move the skeleton to fit the character body and limbs as accurately as you can.
Still in pose mode, parent mesh to armature with 'automatic weights'.
Then, if you return to rest pose, the mesh will follow.


Hope this helps